1. Field of the Invention
This invention relates in general to refrigerant expansion devices for use in heat pump systems. More specifically, this invention relates to an expansion device that has a variable expansion area operated by the pressure differential between the high and low sides of a heat pump system and which is capable of providing an optimum expansion area in both the cooling and defrost modes of operation.
2. Description of the Prior Art
Conventional heat pumps include a refrigeration circuit with a compressor and indoor and outdoor heat exchanger coils which function alternately as a condenser and an evaporator in response to a thermostat controlled valve which reverses the direction of refrigerant flow through the circuit between heating and cooling cycles. During cooling cycles the indoor coil functions as an evaporator, absorbing heat from indoor air, and the outdoor coil functions as a condenser, rejecting heat into the outdoor air.
During heating cycles the outdoor coil functions as an evaporator absorbing heat from the outdoor air, and the indoor coil functions as a condenser rejecting that heat to the indoor air for comfort heating. During the time outdoor temperatures are around 45 degrees, and colder, moisture from the outdoor air is collected onto the outdoor coil fins in the form of frost. The frost accumulates progressively in thickness on the fin surfaces thereby reducing heat transfer by blocking air flow therethrough, and by its insulating effect on the fin surfaces.
The frost accumulation is periodically removed by temporarily operating the heat pump in a cooling cycle wherein hot gas discharged from the compressor is circulated to the outdoor coil to heat it for frost removal. A defrost cycle is functionally a temporary cooling cycle. It is common practice to initiate defrost cycles by automatic means responsive to the thickness of frost accumulation, or by an interval timer. Termination of defrost cycles are typically caused by a thermostat which senses temperature rise of the outdoor coil, or its condensate, indicating completion of frost removal.
Each heat pump coil is usually provided with its own expansion device operative during the time the coil is serving as an evaporator. The device serving the outdoor coil, in heating cycles, provides for metering liquid refrigerant to efficiently meet the circumstances of evaporation during a range of cold outdoor winter temperatures. For example, at a winter ambient of 25 degrees F. the evaporating pressure in the outdoor coil would be approximately 35 PSIG, and the condensing pressure in the indoor coil 195 PSIG, establishing a pressure difference across the expansion device of 160 PSI.
The expansion device serving the indoor coil during the summer cooling cycles is selected to meter liquid refrigerant to the indoor coil during a range of summer cooling temperatures. As an example, at 85 degrees F. ambient, the condenser pressure in the outdoor coil would be approximately 250 PSIG, while the evaporating pressure in the indoor coil would be in the range of 72 PSIG, establishing a pressure difference across the expansion device of 178 PSI.
When a defrost cycle is initiated, refrigerant flow is reversed and circulation of refrigerant in the cooling direction is caused to occur for a set time period, or until a set temperature at the outdoor coil, for example: 80-85 degrees F., is reached. During defrost operation energy penalties are paid which reduce the operating efficiency of the heat pump system. Specifically, during defrost, electrical energy is being consumed by the refrigeration system to defrost the coil with no resultant mechanical heat from the heat pump system being transferred to the heated area. During defrost, heat is actually being removed from the heated area and transferred to the outdoor coil to melt the frost. Further, during the time of defrost, generally, an electric resistance back up heating system installed in the duct work is actuated to maintain the heated space at a desired comfort level. As a result, it is evident that, it is extremely desirable to minimize the defrost time of a heat pump system in order to increase the operating efficiency of the system. One common measure of the efficiency of a heat pump system is the Heating Seasonal Performance Factor, commonly referred to as HSPF. This term is defined by the U.S. Department of Energy as "The total heating output of a heat pump during its normal annual usage for heating divided by the total electric power input during the same period."
Accordingly, since the electrical input is far more efficient when providing heat through the heat pump system, it is extremely desirable to minimize the length of the defrost cycle.
Typical heat pumps are designed with greater outdoor coil volume than indoor coil volume. This is done to maximize cooling performance which is typically the major selling feature or purpose of the heat pump. As a result, the circulated refrigerant charge quantity is greater during the cooling cycle than the heating cycle.
Upon initiation of defrost, a heat pump is shifted from a heating cycle to a cooling cycle. One factor affecting the length of the defrost cycle is the time required to get into circulation, the proper amount of refrigerant charge to maximize heat transfer from the conditioned space to the cold frosted outdoor coil. When a defrost cycle is initiated, by establishing a temporary cooling cycle under typical winter ambient conditions, the condensing pressure in the outdoor coil is the maximum pressure available for delivering refrigerant from the outdoor coil to the indoor coil through the cooling expansion device. Under such circumstances, the cooling expansion device exhibits a high resistance to flow thereacross because it is designed to control refrigerant flow under a pressure differential in the range of 178 psi as shown in the example given above. Under such circumstances, the compressor is usually required to reduce the pressure in the indoor coil to less than zero to establish a pressure differential capable of feeding the indoor coil. In some systems, under certain circumstances, a satisfactory defrost cycle cannot be accomplished with the cooling expansion device serving as the defrost expansion valve.
It has been recognized that during defrost operation, the difference between the high and low pressure in a heat pump system is so small that optimal refrigerant circulation is not guaranteed. One approach to solving this problem has been to provide a solenoid actuated bypass arrangement which provides a large, very low resistance, path bypassing the cooling expansion valve during defrost operations. The theory behind such a bypass valve is to "carry out defrosting as quickly as possible". In practice, however, it has been found that upon initiation of defrost, a low resistance bypass, which allows refrigerant, previously stored in the accumulator during the heating cycle, to be quickly withdrawn and put into circulation where it may deliver heat to the outdoor coil, does not necessarily reduce defrost times. It has been found that, while such a system may quickly melt the frost on the coil, the low resistance bypass to the expansion valve is not conducive to raising the temperature of the outdoor coil to the desired defrost termination temperature which may be as high as 80.degree. to 85.degree. F.
One proposed solution to this problem is set forth in commonly assigned U.S. Pat. No. 4,429,552, "Refrigerant Expansion Device" to Wayne R. Reedy. The '552 patent recognizes that the low pressure differential upon initiation of defrost results in less than a desirable amount of refrigerant flow through the refrigerant expansion device. An expansion device made from a shape memory alloy is provided which is capable of providing two different expansion bores, depending on the temperature of the refrigerant flowing through the device. A larger bore size serves as the expansion device during the first portion of the defrost cycle and the device then changes to a smaller bore size responsive to an increase in temperature later in the defrost cycle.
A refrigerant expansion device that is capable of responding to certain pressure and flow conditions to provide an optimum expansion area within the device for such pressure and flow conditions is disclosed and claimed in commonly assigned U.S. patent application, Ser. No. 473,481, filed on Feb. 1, 1990, entitled, "Variable Area Refrigerant Expansion Device".
The '481 application discloses a refrigerant metering device having a housing with a flow passage extending therethrough. Mounted within the housing is a piston having a flow metering port extending axially therethrough. The piston is mounted such that it is movable within the flow passage. An elongated member is also provided within the housing and extends into the metering port of the piston. The elongated member and the metering port cooperate to define a flow metering passage between them. The elongated member is configured such that the cross-sectional area of the flow metering passage varies in relation to the position of the elongated member to the flow metering port. Means are provided for supporting the elongated member within the housing and for controlling the axial position of the elongated member and the piston with respect to one another as a function of the differential pressure across the flow metering piston.